1. Field of the Invention
This invention relates to improved reinforced concrete piles which are more easily severed than conventional piles.
2. Description of the Prior Art
Where satisfactory bearing soil is not present at a reasonable depth, and the open-caisson method is for any reason impracticable, foundation piles are often used. Wood piles are subject to destruction by various causes. Concrete piles are less destructible and, hence, are adapted to many conditions. While a wood pile is usually designed to carry 15 to 20 tons per pile, a concrete pile is capable of carrying 25 to 60 tons.
Piles may be driven by the drop hammer or steam hammer methods. The steam hammer, with its comparatively light blows delivered in rapid succession, is of advantage in a plastic soil, the speed with which the blows are delivered acting to prevent the readjustment of the soil. It is also of advantage in soft soils where the driving is easy, but a light hammer may fail to drive a heavy pile satisfactorily. A water jet is sometimes used in sandy soils. Water supplied under pressure at the point of the pile through a pipe or hose run alongside it erodes the soil, allowing the pile to settle into place. To have full capacity, jetted piles are driven after jetting stops.
Piles may obtain their supporting power from friction on the sides or from bearing at the point. In the latter case, the bearing power may be limited by the strength of the pile, considered as a column, to which, however, the surrounding soil affords some lateral support. In the former case, no precise determination of the bearing power can be made. Many formulas have been developed for determining the safe bearing power in terms of the weight of the hammer, the fall and the penetration of the pile per blow, the most generally accepted of which is known as the "Engineering News" formula. This formula and similar ones are based on the determination of the energy in the falling pile hammer head and, from this, the pressure which it must exert on the top of the pile. The piles are then driven to a depth calculated as necessary to give the bearing power for which they were designed.
Concrete piles may be divided into two classes, those which are molded in place and those which are precast, cured and driven. Piles of both types, longer than 100 feet, have been driven. In one well known pile of this type (Raymond), a thin steel sheet is fitted over a tapered mandrel before driving. This shell, which is left in the ground when the mandrel is withdrawn, is filled with concrete. Another well known pile (Simplex) of the molded-in-place type uses a hollow cylindrical mandrel which is filled with concrete after having been driven to the desired depth and raised a few feet at a time, the concrete flowing out of the bottom and filling the hole in the earth. Precast and molded-in-place piles may be reinforced with steel. Only steel-reinforced piles are of interest in this invention. Prestressing of precast concrete piles gives greater assistance to handling and driving stresses. Jetting is used extensively in placing precast concrete piles, but most are driven into place. Jetting is often not practical or safe in city work, because of danger to the foundations of adjoining buildings. Driving by hammer necessitates a cushioned driving head.
Concrete piles are preferably spaced not closer than 3 feet on their centers. If driven closer than this, one pile is liable to force another up. Piles in a group must not cause excessive pressure in the soil below their tips.
After the piles are cut off, either conventionally or in accordance with this invention, they generally are capped with concrete.
The driven concrete piles of this invention are prestressed with steel cables, so that they can withstand the stresses caused by driving, as well as the deadload of the structure the piles support. The prestressing of the concrete is accomplished by stressing the steel between anchorages before casting the pile and then, after the concrete pile has hardened, transferring the load to the concrete by anchoring the ends of the steel in the pile. Very high strength steel bars, wire, or cable are used in order to keep down the space required by, and the weight of, the steel, and to minimize the relative losses in steel stress due to shrinkage and the creep of the concrete upon relaxation of the steel. Frequently 4 steel cables are used in concrete piles, but this number can vary, depending upon the diameter of the pile and the expected bearing load. Generally, the steel is spaced apart four times the diameter of the individual wires or three times the diameter of the strands and at least one and one-third times the maximum size of the concrete aggregate.
In employing steel reinforced concrete piles, there is a continuing problem caused by the differential depths to which the piles can be driven. Thus, for example, two apparently identical piles, driven only 50 feet apart, may have final driven depths which differ by several feet. For this reason, steel reinforced concrete piles are manufactured in standard lengths for a given piling, which length is usually in excess of the expected depth requirement. After the piles are driven, the portions of the pile projecting above the ground are all cut off in the same plane (usually at ground level). With conventional steel reinforced concrete piles, the reinforcing cables or wires, which extend the length of the pile, are several inches below the pile side surfaces. In order to cut such a pile at the desired point, it is necessary to drill through the concrete until a cable or wire is reached, after which the cable is cut with any conventional method. This process must be repeated for each one of the cables, the remaining (concrete) portion of the pile being snapped off with a crane. Piles are usually cut in groups, and on the average, a crew of two requires an eight hour day to cut twelve piles, each having four reinforcing cables. The actual drilling may require as much as fifteen minutes per cable, after which the cable must be cut.
Various means are employed to cut the cables, usually electric arc cutting. Two types of metal cutting allied to welding are frequently used, namely, oxygen cutting and electric arc welding. However, for various reasons, electric arc welding (cutting) is much preferred.
Oxygen cutting is based on the rapid, exothermic oxidation of iron when heated to above 1500.degree. F. in the presence of oxygen. The process is therefore, usable with ferrous metals, especially steel bars and the like. Instead of a welding torch, a cutting torch is applied to the gas hoses, and the oxygen-supply pressure is increased. The tip of the cutting torch contains a ring of small orifices for the preheating flame and a central orifice for the oxygen jet. When the steel is suitably heated, the oxygen is turned on, "burning" a clean, narrow cut as the torch advances. For preheating, oxygen and acetylene are commonly used; other fuel gases are hydrogen, propane, natural gas, etc. The speed of cutting using oxyacetylene varies from about 8 to 12 inches per minute for a manual cut through 1-inch thick steel to about 5 to 7 inches per minute for a manual cut through 2-inch thick steel. Four inch thick steel will cut at about 4 to 5 inches per minute manually.
Arc cutting processes rely on electric arc heat to melt a path through the metal and are, therefore, capable of cutting non-ferrous as well as ferrous metals. As in welding, the arc is established between an electrode forming one terminal of an electric circuit and the workpiece forming the other terminal. The cable being the workpiece, it must be grounded.
Various means are employed to flush out the molten metal from the cut. Any suitable method can be used in cutting pile cables, including air carbon-arc cutting, in which the electrode holder is equipped to direct a jet of compressed air in line with the electrode so as to blow away the molten metal.
The most closely related prior art to this invention known to the inventor is as follows.
U.S. Pat. No. 812,223 discloses reinforced concrete piles in which the cables are held together by ties. The drawing of FIG. 1 and the description on page 1, lines 34-39, clearly indicate that the twist of the ties project inwardly toward the center of the pile.
U.S. Pat. No. 1,257,835 relates to reinforced concrete piles especially designed for use in seawater. FIG. 3 might seem somewhat similar to the present invention, however, study indicates that FIG. 3 is merely an end view of FIG. 1, and indicates that the steel cables taper inwardly toward the center of the pile near the top. The purpose of this inward taper is to put an added concrete layer around the steel cables for corrosion protection. This same purpose is accomplished in FIG. 4 by adding more concrete around the upper portion of the pile.
U.S. Pat. No. 1,165,134 relates to prefabricated reinforced concrete piles which have rods running throughout the entire length as well as tie rods. In none of the piles (see FIG. 4) are there any outwardly projecting tie rods.
U.S. Pat. No. 2,355,190 relates to prestressed reinforced concrete piles. In these piles, the steel reinforcing rods do not go through the entire pile but, instead, end somewhat short of the top. The purpose of this construction is to prevent corrosion by seawater.
U.S. Pat. No. 3,501,920 discloses reinforced concrete piles which are in the form of tubes having a cable spirally wrapped around the tensioned reinforcement cables. From the drawings, particularly FIGS. 2, 3 and 6, it is quite apparent that this idea is not closely related to the present invention.